Multilayer film structures containing barrier films are frequently used in the food packaging industry for products such as snack foods, meats and cheeses. In these applications, it is desirable for the multilayer structure to demonstrate good adhesion characteristics and clarity. Inferior adhesion properties can result in de-lamination of the film layers in the manufacturing process or in the end-use product. Clarity is an important aesthetic property having direct influence on consumers. Tie layers are often incorporated in multilayer film structures to adhere dissimilar layers which cannot be bonded directly, however, conventional efforts to improve adhesion provided by the tie layer, such as by increasing the level of grafted material in the tie layer composition, can result in a corresponding deterioration in clarity. This unfortunate result can sometimes occur in applications using an ethyl vinyl (ethylenevinyl or ethylene-vinyl) alcohol copolymer (“EVOH”) barrier layer, where undesirable chemical interaction between the maleic anhydride functionality of the tie layer resin and the EVOH functionality of the barrier layer result in a distorted appearance in the product film, typically described as “orange peel” or “grainy.” Clarity can also be affected in the process by which film layers are coextruded, when differences in the viscosities of the molten resins making up adjacent layers cause stress at the layer interface, leading to weavy-type flow instability.
Various adhesive mixtures have been described in U.S. Pat. Nos. 4,087,587; 4,298,712; 4,487,885; 4,774,144, 5,367,022, 5,439,974, 5,525,672, 5,597,865, 6,716,928, 6,838,520, 7,125,929 and Pat. Appl. Pub. Nos. 2007/0054142, 2008/0032148 and 2008/0163978. Nevertheless, a continuing need exists for tie layer adhesive compositions in multilayer film applications that provide improved adhesion, while minimizing deterioration in, or improving, film clarity. It has unexpectedly been found that mixtures of an olefin polymer base resin, modified polyolefin and a paraffinic liquid hydrocarbon provide a superior balance of adhesion and clarity in tie layer compositions.